I am looking at driving an electrostatic with a digital amp so that the ESL acts as a DAC.Does anyone have experience of high voltage PCM or am I on my own?:)

R. McAnally

26th January 2002 02:35 AM

"Digital" amps are somewhat of a misnomer, as the output is actually PWM, not PCM. People like to call them digital due to the fact they are class D, but it really has nothing to do with digital PCM. Some chips can accept a digital signal, convert to analog (by modulating the PWM output directly) all on one chip, but better results are obtained by using a different amplifier class with external DAC. Class D is probably the worst choice for sound quality (especially high frequencies) due to switching limitations of the devices.....

-Randy

Won

26th January 2002 09:16 AM

This is not entirely true. There exist several amplifiers with fully digital PCM->PWM conversion. These amps are more properly called "digital" because there is no linear gain stage. The closest thing to a DAC in this situation is the output low pass filter.

-Won

R. McAnally

27th January 2002 12:01 AM

Thats what I was trying to get at... amps that convert PCM to PWM are still class D at the output.

Dave

27th January 2002 04:18 AM

R. McAnally,

There is a difference between Class D and a true Digital Amp.

Class D generally employs feedback in a similar fashion to a normal analog amp and has an analog input signal - you use a conventional DAC with it.

A true digital amp uses no feedback only DSP techniques to convert PCM into PWM.

The output section of an amplifier is what determines its class. Anything with a PWM output section is class D, feedback or no feedback, DSP or not. I agree the amplifier topology is digital, but the OPS remains class D, no matter what comes before it.

It doesn't matter how the amp creates the PWM waveform, it is still class D. It could use DSP, analog, or dirty diapers... makes no difference.

The amp you are speaking of is a digital "class D" amplifier. The digital processing and class D output is not the same thing.

hifiZen

6th February 2002 10:55 AM

...getting back to the original topic, you'd probably be best off trying to build a conventional low-voltage "digital" amplifier and use a step-up transformer to get the voltage up to ESL drive levels. This is because of the switching characteristics of the output transistors.

The primary limitation of class-D amplifiers, regardless of whether the driver stages are digital or analog, is the accuracy with which the output transistors can be switched on and off rapidly and repeatably. In general, it is easier to switch lower voltages on and off quickly, and so the timing accuracy and switching speed (which ultimately determines the PWM output stage's performance and efficiency) is better. Thus, it makes more sense to use a step-up transformer and switch at a lower voltage where there are more devices to choose from and better performance can be extracted from those devices. In addition to that, the transformer may provide some measure of ultrasonic filtering, which is especially necessary after a switching output stage.

Digital amplifiers are basically just gigantic power DACs, and they usually produce either a 1-bit pulse output stream or a PWM (pulse width modulated) pulse output stream. Either way, you've got a tremendous amount of full-power HF energy to get rid of, and my suspicion is that ESL speakers won't take too kindly to that level of high power HF drive. So, while it's a nice idea to drive an ESL panel directly from a high-level digital amp output, in reality you will need some filtering there regardless of whether that comes in the form of big RLC networks, or a step-up trannie + passive filter or whatever. The final nail in the coffin (IMHO) is that there don't seem to be any existing designs for high-voltage digital amplifiers. So, while this technology is still in it's infancy, it would seem wise to stick with proven designs.

Given the level of performance of today's digital amplifiers, I believe that a small-signal DAC can do a much better job of the digital-to-analog conversion. I feel that a high quality conventional DAC and amplifier can easily outperform the best digital amplifiers, at least until the technology advances by a significant margin. It took a long time to get small-signal DACs to their present state of refinement, and the challenges facing digital power amplifier designers are still very large. So, you may want to make a careful assessment of your design goals before diving heads-first into a digital amp project, not that I'm trying to dissuade you...